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3.6 Geotectonic interpretation and correlation of the magmatic belts
In the following, the detailed geochemical and geochronological analyses are used
to develop a regional syntheses of the geotectonic setting in which the magmatic belts
of the southern Tien Shan and eastern Pamirs evolved (Fig. 3.11 and 3.16).
South Tien Shan
In the South Tien Shan (STS), the determined Late Palaeozoic (~277 Ma) granites
are interpreted either as mature arc-related intrusions or syn- to post-collisional
granites. The intrusions correspond westward along strike to the Turkestan Alay and
South Gissar zones at the northern margin of the Tadzhik basin (Fig. 3.11, 3.16). Mid-
Carboniferous volcanic rocks of the south Gissar range are interpreted as island arc
tholeiites (Portnyagin et al. 1974). The volcanic rocks are spatially associated with
granitic and granodioritic intrusions of the same age. The whole rock assemblage is in
fault contact to the south with sheared and thrusted blocks of carbonates, turbidites
and olistostromes, clastic molasse and a basal ophiolite melange. Portnyagin et al.
(1974) interpreted this rock association as the “south Gissar marginal suture”.
Northward increasing differentiation of the igneous rocks of the STS, which testifies for
a north-dipping subduction zone, can not only be inferred from this study, but also
across the Gissar range (Leith 1982).
In a section across the easternmost Chinese South Tien Shan (Fig. 3.11), Allen et al.
(1993) report the deposition of a stable marine platform on the northern margin of the
Tarim block, developed over Precambrian continental basement. Through much of the
Late Palaeozoic, the same region was a north-facing passive continental margin
(Windley et al. 1990, Graham et al. 1990) similar to the Gissar region at that time. The
destruction of the passive margin must have been completed by the end of the Early
Permian, as the final episode of marine sedimentation occurred at this time. The
closure of the oceanic basin resulted in the collision of the Tarim block with the Central
Tien Shan along the southern Tien Shan suture (Allen et al. 1993). Regional granitoids
belong either to volcanic arc or syn- to post-collisional settings. Limited regional
knowledge does not allow a clear distinction between a supra-subduction or syn- to
post-collisional origin for these granites (Allen et al. 1993), a situation very similar to
the results of this study. Undeformed Late Palaeozoic calc-alkaline lamprophyre dykes
intrude the Kyrgyz South Tien Shan, the Gissar range and the southern side of the
southern Tien Shan suture in the Chinese Tien Shan (Mogarovskiy 1986, Allen et al.
1993). Allen et al. (1993) tended towards an interpretation of the dykes following